1. Field of the Invention
The present invention relates to a bell signal generating circuit, and more specifically to a bell signal generating circuit for generating a bell signal in differential manner.
2. Description of the Prior Art
In general, a key service unit or a PBX (private branch exchange) for key telephone sets is provided with a bell signal generating circuit 10 as shown in FIG. 1. The bell signal generating circuit 10 is composed of a pulse signal: oscillator 11, a driver circuit 12, and a step-up transformer 13. The pulse signal oscillator 11 generates a low-frequency (20 Hz) pulse signal of 5 Vp-p. The output of the pulse signal oscillator 11 is supplied to the primary winding 13a of the step-up transformer 13 via the driver circuit 12. The secondary winding 13b of the step-up transformer 13 is connected to a dc voltage supply 14, so that a dc voltage can be superposed upon the pulse signal. As a result, a step-up pulse signal of 160 Vp-p can be obtained between two output terminals 15a and 15b of the step-up transformer 13, as shown in FIG. 2.
In the bell signal generating circuit 10 as described above, however, since the pulse signal of 5 Vp-p is stepped up in voltage to the pulse signal of 160 Vp-p, there exists a problem in that the size of the step-up transformer 13 is larger and therefore a large mounting space is inevitably required.
In addition, since the bell signal generating circuit 10 generates a square pulse signal, there exists another problem in that the key telephone set cannot recognize the generation of the bell signal, reliably.